She's 19 meanwhile, he's 25. While he was over 40 in his previous he reincarncated to this fantasy world with a physical appearance of a 25 y/o and mentally he's technically only over 40 since he didn't experience the 25 years he got reborn in.
Not sure why you guys complaining like some hoes.
However, aside from that, I predict that there wont be any potent romance for a 'while' if not anything at all. If I am wrong, you can gladly correct me.